Support using pybind from the system.

This should make packaging easier and more streamlined
to the process on some Linux systems.

python/CMakeLists.txt

@@ -1,4 +1,12 @@
-if(NOT TARGET pybind11)
+find_package(pybind11 2.2 CONFIG)
+
+if(NOT pybind11_FOUND)
   add_subdirectory( pybind11 )
+  # SYSTEM is supported for embedded pybind source (>= 2.3.0)
+  set(PYBIND11_SYSTEM "SYSTEM")
+elseif(pybind11_VERSION VERSION_GREATER_EQUAL "2.3.0")
+  # SYSTEM is only support for pybind >= 2.3.0 (Unfortunately querying the
+  # version does not work. AT least we know the embedded one.
+  set(PYBIND11_SYSTEM "SYSTEM")
 endif()
 add_subdirectory( simulators )

python/simulators/CMakeLists.txt

@@ -4,7 +4,8 @@
 #       find_package(PYTHON) to define variables like
 #       ${PYTHON_EXECUTABLE}
 #
-pybind11_add_module(simulators simulators.cpp SYSTEM)
+
+pybind11_add_module(simulators simulators.cpp ${PYBIND11_SYSTEM})
 
 set_target_properties( simulators PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${PROJECT_BINARY_DIR}/python/opm2 )
 
@@ -26,6 +27,9 @@ file( COPY ${PROJECT_SOURCE_DIR}/python/test_data
       DESTINATION ${PROJECT_BINARY_DIR}/python)
 
 if(OPM_ENABLE_PYTHON_TESTS)
+  if(Python3_EXECUTABLE AND NOT PYTHON_EXECUTABLE)
+    set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
+  endif()
   add_test(NAME python_tests
       WORKING_DIRECTORY ${PROJECT_BINARY_DIR}/python
       COMMAND ${CMAKE_COMMAND}